Dear student
The answer lies in structure.According to bent’s rule more electronegative element occupies axial postion in sp3d hybridisation.

Thus dipole moment cancels out in PCl3F2 whereas it is non-zero in PCl2F3.
						

							
Dear student,
Please find the answer to your problem below.
 
It is to be known that both the mentioned compounds have sp3d hybridisation.
Now we know that more electronegative element occupies axial postion in sp3d hybridisation, which in our case is F.
 
Hence, In PF2Cl3, both the F occupies the opposite axial forces and the 3 Cl lies in the same plane inclined equally to each other. Due to which the net dipole moment becomes 0 as all the dipole moment gets cancelled out.
While In PCl2F3 both axial positions are occupied by F and one planar position is occupied by the third F. The two Cl atoms also lie in the planar position. Now due to this unsymmetry and the fact that Cl and F both have different individual dipole moment, PCl2F3 comes out to have a net resultant dipole moment.
 
Thanks and regards,
Kushagra